This invention relates to a monitor for a receiver. The receiver may be that used in a repeater.
Monitors are highly important for receivers. For example, it is necessary in a high-reliability communication system comprising a pair of terminal equipments at each point of the system to switch a working equipment over to a stand-by equipment as soon as a trouble is detected in the former. On detecting such a trouble, a monitor plays an important role. In a nonstationary communication system, a monitor is resorted to on judging whether or not a communication channel is duly established. In a communication system having rather unstable transmission paths, it is the practice for a receiver to send a request for retransmission back to a transmitter when an objectionable state of reception is found at the receiver. A monitor is used to monitor the state of reception. In a burst communication system where pieces of information for synchronization, such as those for the carrier signal and clock pulses, are collected at a receiver from the signal supplied thereto, it is necessary to derive the information pieces as rapidly and precisely as possible. Inasmuch as the rapidity and precision are inconsistent requisites in this case, the practice is to switchover the modes of operation of the synchronization information extracting circuit from that for initial operation to that for stationary operation. A monitor is used to judge the instant for the switchover.
In all examples mentioned above, a monitor detects the state of reception at a receiver. It may be said that the state of reception finally means the signal-to-noise ratio of the receiver output signal in analog communication and the rate of occurrence of errors in the receiver output codes in digital communication. In order to monitor, for example, the rate of errors, it is mandatory for the monitor to know original codes sent to the receiver through transmission facilities. This, in turn, requires specific codes or words for the check purpose to be transmitted in a time division fashion and accordingly reduces the efficiency of communication. In addition, some appropriate length of time is necessary to detect a slight change in the rate of errors, which is very small under practical circumstances. It is therefore for the monitor not to directly monitor the rate of errors but indirectly with reference to a change in a certain quantity that has a relation with the signal-to-noise ratio or the rate of errors. In carrier transmission, the quantity may be a phase shift or a phase jitter of the local carrier signal produced in the receiver, a phase shift or a phase jitter of the likewise produced clock signal, the signal-to-noise ratio of the receiver input signal, the intersymbol interference appearing in the receiver input signal, a shift in the gain control circuit of the receiver, a shift in threshold levels used in the receiver, a shift in the signals appearing in certain units of the receiver, or the like. It is desirable for the monitor to monitor these quantities as much as possible to derive a monitor output signal that has direct connection with the signal-to-noise ratio of the receiver output signal or the rate of errors in the receiver output codes.